It sounds like a triggered event rather than an action.
–
SevenSidedDieMar 10 '13 at 4:58

I think it's a holdover from earlier editions where there wasn't a rigid action economy, and the rules would just say "do this under this condition." It's more like one of those AD&D rules than a standardised 3.x framework rule.
–
SevenSidedDieMar 10 '13 at 5:49

in addition to a natural attack as an Attack Your Opponent grapple action, only if you were grappling at the start of your turn.

(Intermezzo: Not all monsters with Improved Grab allow you to use Rake. In the Monster Manual the monsters that do are: Hellcat, Leonal, Leopard, (Dire) Lion and (Dire) Tiger.
Examples of monsters that have Improved Grab and Rake but don't allow this are Annis and Behir.)

Some of these actions take the place of an attack (rather than being a
standard action or a move action).

This is the case with Rake for Attack Your Opponent and arguably also for Pounce and Improved Grab, so I'd say a rake attack is a special attack that is part of an action and not an action itself. Compare this also to the extra attacks granted by the Two Weapon Fighting feat or the Haste spell.

-1: Rakes are not made instead of normal natural attacks, they are made in addition to normal natural attacks. To quote the SRD link you give, "A creature with this special attack gains extra natural attacks when it grapples its foe....a monster with the rake ability usually gains two additional claw attacks..."
–
DuckTapeAlMar 10 '13 at 11:35

This one is not so black & white, it is a situational dependent answer.

Attack Action

A monster with the rake ability must begin its turn grappling to use its rake—it can’t begin a grapple and rake in the same turn. Source (Exception noted below)

Triggered
Note: Many creatures with the rake ability will have either Improved Grab or Pounce

Improved Grab: (EXCEPTION to grapple "Attack Action" requirement) To use this ability, (the creature) must hit with its (natural) attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can rake.

Pounce: When a creature with this special attack makes a charge, it can follow with a full attack—including rake attacks if the creature also has the rake ability.

I disagree that the rules are situationally dependent vis-a-vis what kind of action a rake is. No matter how you do it, rake attacks are attack actions made as part of a full attack action(or standard action attack, if you have Improved Grab). There is no such thing as a "triggered action" in 3.5, only move, standard, full, swift, and immediate. Rake is none of these, it is only part of another attack action. This is just semantics, however. I only bring it up because I see the rules-lawyering tag.
–
DuckTapeAlMar 10 '13 at 8:09

Rake is both a special attack and a natural weapon certain creatures have.

The special attack allows you to attack with your rake natural weapons (while the description calls for "two additional claw attacks" the Pounce special attack description a little above on the same page clarifies the "claw" word to be color only.) if you start your turn grappling an opponent.

A rake attack is an extra natural attack that is only useable under certain conditions.
These conditions include making a charge with the pounce special attack and starting a grapple thanks to improved grapple.